1. Field of the Invention
The present invention relates to coil devices, and particularly relates to a coil device such as a transformer and a common-mode choke coil.
2. Description of the Related Art
Conventional coil devices includes a coil device (conventional art 1) shown in FIGS. 7A and 7B, and such a coil device is disclosed in Japanese Unexamined Patent Application Publication No. 8-203737. This coil device is a surface mounted type and is for high frequency use. The coil device has a laminated body 51 including an insulator, in which two spiral coils 52 and 53, magnetic substrates 54 and 55, and external electrodes 56 and 57 are arranged. The spiral coils 52 and 53 face each other with a portion of the insulator disposed therebetween, are sandwiched between the magnetic substrates 54 and 55, and are connected to external electrodes. In FIG. 7B, only the spiral coil 52 is connected to the external electrodes 56 and 57. This coil device has various characteristics including compactness, low profile, better high-frequency properties than a laminated body having a ferrite element assembly in which coils are arranged, no difference in inductance caused by a difference in the relative magnetic permeability, and good coupling between coils in a common-mode choke coil.
Another coil device (conventional art 2) having a configuration shown in FIG. 8 is disclosed in Japanese Unexamined Patent Application Publication No. 11-54326. This coil device has upper and lower magnetic substrates 54 and 55, two spiral coils 52 and 53, a laminated body (layered region) 51 having a ring shape, and an adhesive layer (magnetic layer) 58 having a relative magnetic permeability of 1 or more, wherein the laminated body 51 contains the spiral coils 52 and 53 therein and is disposed on the lower magnetic substrates 54, and the adhesive layer 58 is disposed between the upper and lower magnetic substrates 54 and 55.
In this coil device, since the laminated body 51 disposed on the lower magnetic substrate 54 is covered with the adhesive layer 58 having a relative permeability of 1 or more, the lines of magnetic force generated by the spiral coils 52 and 53 form closed magnetic circuits, as shown in FIG. 8. The adhesive layer 58 and an insulator other than a region where coil patterns are located include a material having a relative permeability of 1 or more, and therefore, the degree of electromagnetic coupling between the spiral coils 52 and 53 is increased. Thus, a large inductance can be obtained.
However, in the coil device of the conventional art 1 disclosed in Japanese Unexamined Patent Application Publication No. 8-203737, there is a problem in that a large inductance and miniaturization cannot be achieved simultaneously because the adjustable range of the inductance is limited.
On the other hand, in the coil device of the conventional art 2 disclosed in Japanese Unexamined Patent Application Publication No. 11-54326, the adhesive layer 58 having a relative permeability of 1 or more covers the laminated body 51 disposed on the lower magnetic substrate 54 to increase the inductance. In order to prepare an adhesive layer having a relative permeability of 1 or more, the adhesive layer must include an adhesive material and a magnetic material. In order to obtain a larger relative permeability, the magnetic material content must be very high. However, there is a maximum magnetic material content when the adhesive layer is to have a large relative permeability and a predetermined adhesive force in combination. Therefore, there is a problem in that the product reliability is decreased when the magnetic material content exceeds the maximum value.
Since layered coils are disposed in the adhesive layer having a relative permeability of 1 or more, the inductance is increased in proportion to an increase in the relative permeability of the magnetic layer. There is a problem in that a difference in the relative permeability in the magnetic layer has a strong effect on the inductance.
In order to solve the above-described problems, preferred embodiments of the present invention provide a coil device in which miniaturization and a large inductance are achieved simultaneously, and which has high reliability.
A coil device according to a preferred embodiment of the present invention includes a first magnetic substrate, a laminated body disposed on the first magnetic substrate and including insulating layers, coil patterns, and at least one through-hole, a magnetic layer covering the upper surface of the laminated body, an adhesive layer disposed on the magnetic layer, and a second magnetic substrate disposed on the adhesive layer and bonded to the magnetic layer with the adhesive layer, wherein the insulating layers define an insulator and the coil patterns for defining coils are stacked so that the coils are disposed in the insulator, the at least one through-hole is located at an area where the coils are not located and extends from the upper surface of the laminated body to the first magnetic substrate, the magnetic layer has at least one portion extending through the at least one through-hole to contact the first magnetic substrate, the adhesive layer is nonmagnetic, and the laminated body is sandwiched between the first and second substrates.
In the above-described coil device, the at least one through-hole is located at an area where the coils are not located in the laminated body and extends from the upper surface of the laminated body to the first magnetic substrate, the magnetic layer is disposed on the laminated body, and the magnetic layer has a portion extending through the through-hole to contact the first magnetic substrate. Therefore, a large impedance can be obtained without increasing the coil device size and the magnetic layer is securely joined to the second magnetic substrate with the nonmagnetic adhesive layer located therebetween. Since a very thin adhesive layer is disposed between the magnetic layer and the second magnetic substrate and functions as a nonmagnetic zone, stable inductance characteristics in a higher frequency band can be obtained as compared with a configuration in which a magnetic layer directly contacts the second magnetic substrate.
Usually, it is difficult to reduce the difference in relative permeability of magnetic bodies to the range of approximately xe2x88x9230% to approximately 30%. In a configuration having perfect closed magnetic circuits, the difference in relative permeability of magnetic bodies has a strong effect on the electrical characteristics. However, in the coil device of the present preferred embodiment, the inductance and the impedance are only slightly changed depending on the difference in relative permeability of the magnetic substrates and the magnetic layer. Therefore, the coil device has high accuracy due to a small difference in the characteristics.
In a preferred embodiment of the present invention, the coil device preferably functions as a common-mode choke coil having a configuration in which a plurality of the coils are arranged to face one another in the laminated body with each of the insulating layers being disposed therebetween, and the main portion of each coil and each insulating layer are alternately stacked. The main portion of each coil includes an area except for portions for connecting to the terminal electrodes of the coil.
In this coil device, since the common-mode choke coil has the above-described configuration, a magnetic flux is allowed to converge in the magnetic substrates and the magnetic layer. Since the common magnetic flux generated between a pair of coils facing each other can be increased compared with conventional common-mode choke coils, the degree of coupling between the coils can be increased. Thus, for the electrical characteristics, the impedance in a differential mode can be decreased, thereby reducing the influence on the transmitted waveform.
In the coil device of a preferred embodiment of the present invention, the coils are preferably spiral-shaped and preferably have the through-hole located at the approximate center of each coil.
Since the coils have the above-described configuration and the through-hole extends from the upper surface of the laminated body to the first magnetic substrate, closed magnetic circuits extending from substantially the centers of the coils to the peripheries thereof and further extending to the approximate centers are provided. Thus, when the coil device functions as a common-mode choke coil having a plurality of coils, the degree of coupling between the coils can be increased and a large impedance can be obtained.
In the coil device of a preferred embodiment of the present invention, the coils and/or the insulating layers are preferably formed by a photolithography method.
Since the coils and/or the insulating layers are formed in the above manner, the coils are very fine, thin, and precise. Thus, small high-performance coil devices efficiently providing inductance and impedance can be obtained.
In the coil device of a preferred embodiment of the present invention, the magnetic layer disposed between the first and second magnetic substrates preferably has a relative permeability of about 2 to about 7.
Since the magnetic layer has a relative permeability of about 2 to about 7, the coil device efficiently provides a large inductance and impedance.
When the magnetic layer has a relative permeability of less than 2, the desired inductance cannot be obtained and changes in inductance are increased depending on the difference in relative permeability. In contrast, when the magnetic layer has a relative permeability of more than 7, the inductance can be increased but the magnetic layer cannot have the required adhesiveness because the magnetic material content (magnetic powder content), for example, the magnetic material content in a resin compound, must be significantly increased.
In the coil device of a preferred embodiment of the present invention, the distance between the first and second magnetic substrates is preferably about 70 xcexcm or less, and the adhesive layer preferably has a thickness of about 1 xcexcm to about 5 xcexcm.
When the distance exceeds about 70 xcexcm, the desired inductance cannot be obtained.
When the thickness of the adhesive layer is less than about 1 xcexcm, a large inductance can be obtained but there is a risk that poor adhesion arises because an adhesive layer having such a small thickness cannot accommodate the surface roughness of the laminated body and the magnetic layer. Furthermore, differences in the characteristics are increased depending on changes in the thickness. When the thickness of the adhesive layer is more than about 5 xcexcm, a large adhesive force can be obtained but the inductance is decreased and therefore the effects obtained from the configuration according to preferred embodiments of the present invention are decreased.
In the coil device of various preferred embodiments of the present invention, the magnetic layer and the adhesive layer preferably have a cavity therebetween and the cavity is located at an area substantially corresponding to the through-hole formed in the laminated body in plan view.
Since the cavity is a recessed portion in the magnetic layer and is located between the magnetic layer and the adhesive layer, the volume ratio of the magnetic layer in the laminated body is decreased, thereby reducing changes in the inductance depending on the difference in relative permeability of the magnetic layer or the difference in the state of the magnetic layer portion extending through the through-hole of the laminated body. Therefore, the desired inductance and impedance can be obtained with high accuracy.
In the coil device of a preferred embodiment of the present invention, the depth of the cavity is about 0.2A to about 0.6A, where A represents the distance between the upper surface of the first magnetic substrate and the lower surface of the adhesive layer, wherein the lower surface is an area where the cavity is not located in the upper surface of the magnetic layer.
Since the cavity has such a depth, changes in inductance are small, thereby reducing the difference in inductance.
When the cavity has a depth of less than about 0.2A, the difference in inductance is increased depending on the processing accuracy. When the cavity has a depth of more than about 0.6A, the desired inductance cannot be obtained efficiently.
A coil device according to a preferred embodiment of the present invention includes a first magnetic substrate, a laminated body disposed on the first magnetic substrate and including insulating layers, coils, and at least one through-hole, a magnetic layer covering the upper surface of the laminated body, an adhesive layer disposed on the magnetic layer, a cavity located at an area between the magnetic layer and the adhesive layer, the area substantially corresponding to the through-hole, and a second magnetic substrate disposed on the adhesive layer and bonded to the magnetic layer with the adhesive layer, wherein the insulating layers define an insulator and the coil patterns for forming coils are stacked so that the coils are disposed in the insulator, the at least one through-hole is located at substantially the center of the coils and extends from the upper surface of the laminated body to the first magnetic substrate, the magnetic layer has at least one portion extending through the through-hole to contact the first magnetic substrate and has a relative permeability of about 2 to about 7, the adhesive layer is nonmagnetic, and the laminated body is sandwiched between the first and second substrates.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.